Transverse spatial mode transformer for optical communication

ABSTRACT

A transverse spatial mode transformer for transforming an optical signal between different spatial modes is described. The transformer is based on a spatially selective change of the phase of the optical signal wavefront relative to the initial wavefront. As the phase-adjusted optical signal propagates, the transverse intensity distribution changes to correspond to the new spatial mode. The transformer can be used to change the lower order spatial mode of an optical signal to a higher order spatial mode appropriate for a dispersion compensated fiber optic communication system. The transformer can also be used to change a higher order spatial mode to a lower order spatial mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional U.S. patent applicationNo. 60/079,423 which was filed Mar. 26, 1998, provisional U.S. patentapplication No. 60/089,350 which was filed Jun. 15, 1998 and provisionalU.S. patent application No. 60/091,026 which was filed Jun. 29, 1998 andincorporates by reference U.S. patent application Ser. No. 09/249,830entitled “Optical Communication System with Chromatic Dispersion” filedFeb. 12, 1999 and U.S. patent application Ser. No. 09/249,920 entitled“Apparatus and Method for Compensation of Chromatic Dispersion inOptical Fibers” (now U.S. Pat. No. 6,339,665) filed concurrentlyherewith.

FIELD OF THE INVENTION

The invention relates to fiber optic telecommunication systems and morespecifically to chromatic dispersion compensation in such systems.

BACKGROUND OF THE INVENTION

The tendency of a pulse of light propagating through an optical fiber tobroaden is a result of the fact that different wavelengths of light passthrough the fiber at different speeds. This speed differential whichcauses the pulse to broaden is termed chromatic dispersion. Chromaticdispersion presents a problem in modem optical communication systemsbecause the tendency of light pulses to broaden as they propagate downthe fiber causes the closely spaced light pulses to overlap in time.This overlap can have an undesirable effect since it restricts howclosely spaced the pulses can be. This in turn limits the data bandwidthof the optical fiber.

There are many characteristics of dispersion. First order dispersion isthe rate of change of index of refraction with respect to wavelength inthe fiber. First order dispersion is also referred to as group velocity.Second order dispersion is the rate of change of the first orderdispersion with respect to wavelength. Second order dispersion producesthe pulse broadening. Third order dispersion is the rate of change ofbroadening with respect to a change in wavelength. This is oftenreferred to as the dispersion slope.

Several solutions have been proposed to mitigate the effects ofdispersion in transmission fibers. One technique involves the use of acompensating optical fiber having an appropriate, length and which has adispersion that is opposite to the dispersion characteristic of thetransmission fiber. The result is dispersion in the transmission fiberis substantially matched and canceled by the total dispersion in thecompensating fiber. While this technique offers a solution to thedispersion problem, it may be impractical in actual use because of theattenuation due to the required length of the compensating fiber. Insuch a case, the total transmission length of the fiber is significantlyincreased thereby increasing the signal attenuation in the fiber.Furthermore, it may be difficult to find a fiber of the desired lengthwith the required dispersion properties.

It is also difficult to design a fiber having a changing index ofrefraction across the diameter of the fiber (the fiber index profile)that will compensate simultaneously for the second and third dispersionorders. It is even more difficult to control the material properties ofsuch fibers even in the most accurate fabrication process necessary toproduce such fibers. In addition, the process of fabricating the singlecompensating chromatic dispersion fiber is expensive and generally notpractical.

When a pulse of light is transmitted through an optical fiber, theenergy follows a number of paths which cross the fiber axis at differentangles. A group of paths which cross the axis at the same angle is knownas a mode. Sometimes it is necessary to limit or control the number ofmodes used in a transmission system. The fundamental mode LP₀₁ in whichlight passes substantially along the fiber axis is often used in highbandwidth transmission systems using optical fibers commonly referred toas single mode fibers.

The dispersion properties of high order modes have been investigated atlength. There is a dependence of high order mode dispersion onwavelength and on the properties of the fiber. By properly designing thefiber index profile it is possible to make the dispersion slope bepositive, negative or zero. It is also possible to make the magnitude ofthe dispersion be negative, zero or slightly positive. Using these twoproperties one can either control or compensate for the dispersion inany transmission fiber.

Systems have been developed to take advantage of higher order modes tocompensate for dispersion in a typical optical communication system. Insuch systems it has been necessary to first convert the lower orderfundamental mode of the light to a higher order spatial mode. This isaccomplished using longitudinal mode conversion.

Prior art methods for mode conversion are known as longitudinal modeconversion and are based on introducing a periodic perturbation alongthe fiber axis. The length of each period and the number of periods inthese longitudinal converters must be determined accurately according tothe wavelength, the strength of the perturbation, and the modesinvolved. By constructing a longitudinal mode converter it is possibleto achieve good efficiency in transferring the energy from one mode tothe other in a limited spectral bandwidth. This spectral property hasbeen used in Dense Wavelength Division Multiplexing (DWDM) applicationsin telecommunications for other applications. Unfortunately, thistechnique is accompanied by significant energy attenuation and it cannotbe used over broad spectral bandwidths.

Another deficiency associated with longitudinal mode converters isrelated to the fact that after the conversion, only a single mode shouldbe present in the fiber. It can be difficult to discriminate betweendesired modes and undesired modes having almost the same groupvelocities because unwanted modes can appear at the output of theconverter. As the modes propagate, modal dispersion occurs and the pulsebroadens. Generally, longitudinal mode converters introduce significantenergy attenuation and noise. Therefore, a trade-off must be madebetween having broad-spectrum capability and the demand for convertingthe original mode to a pure, single, high-order mode.

One such longitudinal mode converter is discussed in U.S. Pat. No.5,802,234. Here, a single mode transmission fiber carries the LP₀₁ to alongitudinal mode converter. Before conversion in this system, however,it is necessary to couple the single mode transmission fiber to amultimode fiber while maintaining the signal in the basic LP₀₁ mode.This coupling is typically difficult to achieve without signaldegradation and any misalignment or manufacturing inaccuracies canresult in the presence of higher order modes. It is desirable that onlythe LP₀₁ mode propagate initially in the multimode fiber in order toavoid significant noise that degrades the system performance andtypically such coupling results in the propagation of additional modes.

The present invention overcomes the disadvantages of longitudinal modeconverters and previous attempts to control dispersion in a fiber opticsystem.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus and method fortransforming an optical signal between different spatial modes. Theapparatus and method are based on a spatially selective phase change ofthe optical signal wavefront relative to the initial wavefront. As thephase-adjusted optical signal propagates, the transverse intensitydistribution changes to correspond to the new spatial mode.

The present invention features a transverse mode transformer having anoptical input and a spatially selective retardation element. Theretardation element transforms an optical signal received at the opticalinput from a first spatial mode to a second spatial mode. Theretardation element can be a phase plate, a lens, a mirror, a grating,an electro-optic element, a beam splitter or a reflective element. Inone embodiment the second spatial mode is of a higher order than thefirst spatial mode.

In another aspect, the invention features a method of spatial modetransformation which includes the steps of providing a spatiallyselective retardation element, receiving an optical signal having afirst spatial mode at the retardation element and spatially retarding atleast a portion of the optical signal to generate a second spatial mode.In one embodiment, the second spatial mode is a higher order mode.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages of the invention may be more clearlyunderstood with reference to the specification and the drawings, inwhich:

FIG. 1 is a block diagram of an embodiment of a typical fiber optictransmission system known to the prior art;

FIG. 2 is a block diagram of an embodiment of the fiber optictransmission system of the present invention including a chromaticdispersion compensation fiber module;

FIG. 3 is a block diagram of an embodiment of the chromatic dispersioncompensation fiber module shown in FIG. 2 showing transverse modetransformers and a chromatic dispersion compensation fiber;

FIG. 4 is a block diagram of another embodiment of the chromaticdispersion compensation fiber module of the present invention showingtransverse mode transformers and two chromatic dispersion compensationfibers;

FIG. 5 is a highly schematic diagram of an embodiment of a transversemode transformer shown in FIG. 3;

FIG. 6a is a block diagram of an alternative embodiment of a fiber optictransmission system of the current invention with the leadingtransmission fiber replaced by a transmission source;

FIG. 6b is a block diagram of an alternative embodiment of a fiber optictransmission system of the current invention with the receivingtransmission fiber replaced by a detector;

FIG. 7a is a graph of the intensity as a function of position along thediameter of a fiber in an ideal case;

FIG. 7b is a graph of the intensity as a function of position along thediameter of the fiber after transformation to the LP₀₂ mode;

FIG. 8 is a graph of the relative energy in the higher order moderelative to the LP₀₁ mode for an element optimized for operation at awavelength of 1550 nm in an ideal case;

FIG. 9 is a block diagram of an alternative embodiment of a transversemode transformer using two phase elements;

FIG. 10a is a highly schematic diagram of an alternative embodiment ofthe present invention showing two chromatic dispersion compensationfibers used for multiple order dispersion compensation;

FIG. 10b is a highly schematic diagram of an alternative embodiment ofthe present invention showing two chromatic dispersion compensationfibers sandwiching a single mode transmission fiber used for multipleorder dispersion compensation;

FIGS. 11a-11 e are graphs of different solution spaces showing relativedesign characteristics resulting from the use of first and second orderdispersion;

FIGS. 12a-12 c are illustrations of alternative embodiments of thetransverse mode transformer shown embedded in a fiber optic transmissionsystem;

FIGS. 13a-13 c are graphs of the amplitude versus position plot of thepulse across the diameter of the fiber before, during and after modetransformation;

FIG. 14 is an illustration of an alternative embodiment of the currentinvention using a polarization beamsplitter and a polarization combiner;

FIG. 15 is a schematic diagram of a single bulk component that can beused to replace the discrete bulk optical components in the embodimentshown in FIG. 14;

FIG. 16 shows a representation of the polarization of propagating modesthrough the element described in FIG. 15,

FIG. 17 shows a representation of the polarization of propagating modesusing a birefringent element;

FIG. 18 is a block diagram of an alternative embodiment of the currentinvention designed to eliminate the sensitivity of the system topolarization mode dispersion by using a circulator and a Faraday mirror;and

FIG. 19 is a block diagram of an alternative embodiment of the currentinvention designed to eliminate the sensitivity of the system topolarization mode dispersion without using a circulator.

FIGS. 20a-20 c are diagrams of alternative embodiments of a transversemode transformer using internal reflection.

DETAILED DESCRIPTION OF THE INVENTION

A typical optical fiber transmission system known in the prior art isshown in FIG. 1. Such a system includes a signal transmitter 2 inoptical communication with a single mode fiber (SMF) 3 which is in turnin optical communication with a signal receiver 4. (Other componentscommon to optical fiber systems, such as amplifiers, circulators,isolators, etc. are not shown.) A signal is transmitted from thetransmitter 2 into the fiber 3 where it propagates some distance.Depending on the length and other properties of the fiber, significantsignal attenuation and dispersion can occur in the fiber. The receiver 4acquires the attenuated signal as it exits the fiber 3.

A basic configuration of the system of the present invention ispresented in FIG. 2. A transmitter 2 transmits an optical signal into acommunication fiber 3. The communication fiber 3 introduces dispersionthat requires compensation. The chromatic dispersion compensation module10 compensates for signal dispersion introduced by the communicationfiber 3 before propagating the signal into a receiver 4.

An embodiment of the chromatic dispersion module 10 is shown in FIG. 3.A signal propagating in a single mode fiber (SMF) 3 enters a modetransformer 28 which converts the basic lower order spatial mode,generally LP₀₁, to a higher order spatial mode, generally LP₀₂, thatpropagates in a special chromatic dispersion compensating fiber 30. Thechromatic dispersion compensation fiber (DCF) 30 is designed tocompensate for the first order dispersion of the signal. A secondchromatic dispersion compensation fiber 31 with different compensationproperties may be coupled to the first chromatic dispersion compensationfiber 30 in order to compensate for dispersion slope as shown in FIG. 4.If required, more than two chromatic dispersion compensation fibers maybe used to compensate even higher order dispersion or alternatively formode filtering applications. Once compensation is complete, the signalis then converted back to the lower order mode by a second modetransformer 28′ and emerges from the chromatic dispersion compensationmodule 10 in the single mode fiber 3′.

The mode transformer 28 of the present invention is a bi-directionaltransverse mode transformer. It can be used to convert a lower orderspatial mode to a higher order spatial mode. Conversely, the sametransverse mode transformer 28 can be used to convert a higher orderspatial mode to a lower order spatial mode. Unlike prior modetransformers which used the longitudinal axis of the fiber to accomplishlongitudinal mode conversion, the present transverse mode transformeruses transverse properties of the wavefront of the light to mode convertby selectively altering the phase of at least one portion of thewavefront. One embodiment of a transverse mode transformer is shown inFIG. 5. A transverse phase element 58 arranged perpendicular to thelongitudinal axis of the fiber is used to accomplish modetransformation. A pulse of light propagates in a single mode fiber 50with a small diameter core 54. The pulse broadens into and expandedregion 56 as it emerges from the fiber. As the pulse passes through thetraverse phase element 58 the phase distribution of the pulse ischanged. The phase element 58 is typically a spatially selective phaseelement, i.e. a spatially selective retardation element. As a result ofthe spatially selective retardation, the phase element 58 alters thephase of points on the wavefront as a function of their transverseposition. A focusing lens 62 focuses the pulse back into the specialchromatic dispersion compensation fiber 64, shown as having a broadercore 66 simply for explanatory purposes. In one embodiment the lens 62is a compound lens. In another embodiment, gradient index (GRIN) lensesare used. The phase element 58 can be any spatially selective phaseelement, including but not limited to, lenses, mirrors, gratings,electro-optic devices, beamsplitters, reflective elements, graded indexmaterials and photolithographic elements.

Phase transformation can be achieved using the properties of sphericalaberration inherent in optical lenses. After a wavefront passes througha lens, it will experience spherical aberration. The resulting distortedwavefront can be used with or without a phase element 58 in thetransverse mode transformer 28 of the present invention to transform thespatial mode of the original wavefront to a higher order spatial mode.

FIG. 6a depicts a system in which a transmission source 24 replaces theoptical fiber 3 shown in the embodiment in FIG. 4. Here the system doesnot require an input transmission fiber and retains all thefunctionality and advantages of the present invention. The transmissionsource 24 injects an optical signal directly into the chromaticdispersion compensation module 10 where it is pre-compensated beforebeing received by the transmission fiber 3′. Precompensation can bedesirable when the transmission fiber 3′ has a known dispersion thatrequires compensation.

FIG. 6b describes a system in which a detector 36 replaces thetransmission fiber 3′ shown in the embodiment in FIG. 4. Here the systemdoes not require an exit transmission fiber 3′ and the functionality ofthe system is not affected. In this case the optical signal propagatesin the optical fiber 3 before being compensated by the chromaticdispersion compensation module 10. Once the signal is down converted bymode transformer 28′, it is detected directly by detector 36. Thismethod can conserve energy since there will not be fiber coupling lossesexhibited before the detector.

The physical mechanism of the transverse mode transformation presentedin this invention is explained with reference to FIGS. 13a to 13 c.(FIGS. 13a to 13 c share the same horizontal scale.) FIG. 13aillustrates the gaussian-like amplitude distribution of mode LP₀₁ in asingle mode fiber, wherein the horizontal axis represents the transverseposition across the diameter of the fiber in arbitrary units and thevertical axis represents the amplitude in arbitrary units. In oneembodiment, the transverse phase element 58 (FIG. 5) introduces a stepfunction to the wavefront 20 of the pulse such that the center region 20a of the wavefront 20 is retarded with respect to the outer region 20 bof the wavefront 20. Therefore, the inner region 20 a and the outerregion 20 b of the wavefront 20 will differ in phase by 180°. Afterpropagation and transformation, the resulting distribution 22 shown inFIG. 13c enters the chromatic dispersion compensation fiber 64 (see FIG.5). More than ninety percent of the transverse intensity distribution inthe LP₀₁ mode (see FIG. 7a) is present in the LP₀₂ mode (see FIG. 7b)after transformation. The remaining energy is distributed among higherorder modes which are not supported by the special chromatic dispersioncompensation fiber 64. Therefore, the fiber will contain substantially asingle high order mode (LP₀₂). The same process, but in the reverseorder, occurs in the second mode transformer 28′ at the opposite end ofthe compensation fiber 64. This technique can also be applied to convertbetween other spatial modes.

One of the advantages of this transverse transformation mechanism is itshigh efficiency over a broad spectrum. FIG. 8 shows the residual energyin the LP₀₁ mode for an element optimized for operation at 1550 nm. Thehorizontal axis represents the wavelength of the pulse in nanometers,and the vertical axis represents the ratio between the energy remainingin the low order mode to the total energy of the pulse. Less than onehalf of a percent of the pulse energy is left in the lowest order modeover greater than 100 nm of spectral range.

In order to further improve the transformation efficiency it is possibleto use multiple phase elements 58′ and 58″ as shown in FIG. 9. The pulseemerging from fiber 50 is collimated by lens 62′, then it passes throughthe two phase elements 58′ and 58″ and is finally focused by lens 62″into a special chromatic dispersion compensation fiber 64. Thistechnique reduces longitudinal sensitivity in the placement of the phaseelements. The design of phase elements 58′ and 58″ can be based on acoordinate transformation technique for converting between spatialmodes. The first phase element 58′ is designed to have local phasechanges across the pulse. Each local phase change redirects (i.e.,steers) a small section of the wavefront 20 to a predeterminedcoordinate on the second phase element 58″. As a result, a predeterminedintensity pattern is generated at the second phase element 58″. Thesecond phase element also induces local phase changes across thewavefront so that the resulting wavefront 20 with predeterminedintensity and phase distributions at the second element 58″ yields thedesired spatial mode.

Another embodiment of the chromatic dispersion compensation module 10 ofthe present invention is shown in FIG. 10a. This embodiment may be usedwith transverse mode transformers 28, but is not limited to their use.Any means that propagates a pulse with a higher order mode into anoptical coupler 6 can use the invention. After the higher order pulsepasses through optical coupler 6, the pulse then enters the firstchromatic dispersion compensation fiber (DCF₁) 8 which is designed tocompensate for the dispersion of the communication fiber 5. DCF₁ 8 isspliced to a second dispersion compensation fiber (DCF₂) 9 through asplice 12. DCF₂ 9 is designed to have minimal second order dispersion atthe point where the dispersion slope is maximum. By properly choosingthe design parameters, a minimal length of DCF₁ 8 and DCF₂ 9 is requiredto compensate for dispersion. DCF₁ 8 and DCF₂ 9 can be designed tooperate with the basic LP₀₁ mode as long as they have differentdispersion characteristics. The order in which DCF₁ 8 and DCF₂ 9 arearranged can be changed. Generally, more chromatic dispersioncompensation fibers are required as the number of dispersion orders tobe compensated increases. The chromatic dispersion compensated pulsepasses into the outgoing optical transmission fiber 5′ at splice 14.FIG. 10b illustrates another embodiment of the invention. A single modefiber 5″ is sandwiched between two dispersion compensation fibers. Anynumber of combinations can be realized without detracting from theessence of the invention.

Graphs of possible solutions using the chromatic dispersion compensationfibers of the present invention are shown in FIGS. 11a-11 e. Thehorizontal axes represent the second order dispersion, and the verticalaxes represent the second order dispersion slope (i.e., third orderdispersion). The dispersion compensation introduced by the chromaticdispersion compensation fibers is presented as arrow 24. FIG. 11arepresents an ideal system, where the desired dispersion solution ispresented as the point 20. By choosing the proper length of chromaticdispersion compensation fiber, the desired results are achieved.Unfortunately, in conventional communication systems it is difficult tochange the relationship between the dispersion orders. Moreover, it isdifficult to even predict this relationship before fabrication of thecompensation fiber is completed. In addition, this relationship variesstrongly according to fabrication processes. Therefore, if the desiredamount of dispersion compensation presented at point 20 is displaced asillustrated in FIG. 11b, it is impossible to achieve the desiredcompensation. It is possible, however, to increase the length of the DCFin order to add length 26 to the arrow 24, so that the actual magnitudeof dispersion is increased and the resulting dispersion 27 willapproximate the desired dispersion 20.

By combining two or more different fibers it is possible to achieve avariety of dispersion properties. The dispersion properties of DCF₁ 8and DCF₂ 9 in FIG. 10a are represented as 32 and 34 in FIG. 11c. Thearea 36 represents the solution space of dispersion compensation whichcan be achieved by proper combination of the two fibers DCF₁ 8 and DCF₂9.

FIG. 11d represents an example of such a combination. Using acombination of two or more DCFs, one can compensate for higher orders ofdispersion. In order to achieve better coverage of the dispersionpossibilities it is desirable to increase the angle between the arrows32 and 34 in FIG. 11c. It is difficult to achieve this result by usingconventional single mode DCFs, however, high order mode-dispersioncompensation fibers (HOM-DCF) can achieve more than 90 degreesdifference between two different DCFs as presented in FIG. 11e. Thissystem is insensitive to the exact properties of the DCFs, becausechanging the length of the fibers can compensate for any deviation inthe result.

FIG. 12a depicts an alternative embodiment of the transverse modetransformer of the present invention and shows a connection, between twofibers, designed to modify the wavefront. Both fibers include a core 11and cladding 12. The face 14 of the transmission fiber 7 can beperpendicular to the face of the dispersion compensation fiber 8 or at asmall angle to the DCF 8 in order to eliminate reflection noise. The endface of at least one of the fibers has a predetermined binary pattern16. The pattern 16 can be etched onto the fiber or be in opticalcommunication with the fiber. The pattern is designed to redistribute agaussian wavefront such as that corresponding to the LP₀₁ mode asdepicted in FIG. 7a to the LP₀₂ mode as depicted in FIG. 7b. In order toachieve an instantaneous change of the wavefront, the height of thebinary pattern is set in one embodiment to 1.5 microns. This height ismuch smaller than the ‘Rayleigh range’, which is approximately 50microns in a conventional fiber. The Rayleigh range is defined as πr²/λwhere r is the radius of the wavefront and λ is the wavelength of thelight.

FIG. 12b depicts an embodiment in which the fibers 7, 8 are in contactwith each other in order to reduce the relative motion and losses. Inone embodiment, the end face of at least one of the fibers 7 and 8 has apredetermined binary pattern 16′. FIG. 12c depicts the same architectureas in FIG. 12b except that a transparent material (for example thecladding itself) fills the gap 17. In this architecture the height ofthe pattern 16″ can be larger. If the relative refractive indexdifference between the filled gap 17 and the pattern 16″ is set to 4%,then the pattern height is set to 13 microns. This height is stillsmaller than the ‘Rayleigh range’.

The width of the wavefront in a fiber is of the order of microns. Sincemodem photolithographic methods can achieved sub-micron resolution,photolithography can be used to create the desired pattern on the faceof the fiber.

Just as photolithography makes it is possible to accurately etch or coatthe desired pattern on the edge of the fiber, multiple lithographicprocesses make it possible to approximate any continuous pattern.Accurate alignment of the fiber core to the desired pattern can beachieved by illuminating the fiber through the core.

Another method for creating a pattern 16 on the end face of a fiber isto attach a short (i.e., a few tenths of microns in length) fiber havingthe desired pattern 16. It can also be done by attaching a long fiber tothe fiber end face and cutting it to the desired length. This method ismore convenient and less expensive in mass production.

An internally reflective spatial mode transformer 190 of the presentinvention is illustrated in FIG. 20a. The gaussian beam emerging fromthe end of a single mode fiber 3 includes a center portion 192 and outerportions 194. The gaussian beam 192 and 194 enters the spatial modetransformer 190 where only the outer portions 194 are reflected from aninternal surface 196 back into the center portion 192 so that theinterference between the portions 192 and 194 results in a wavefrontsimilar to that of the LP₀₂ mode. The resulting wavefront passes throughone or more lenses 198 which couple the wavefront into a high order modefiber 8. The internal surface 196 can be made from a variety ofreflectors including, but not limited to, metallic reflective materialsand refractive index interfaces (e.g., a segment of optical fiber havinga core-cladding interface). FIG. 20b illustrates an internallyreflective spatial mode transformer 190 attached to the single modefiber 3. In another embodiment shown in FIG. 20c, a fiber-based spatialmode transformer 190′ is disposed between the ends of the two fibers 3and 8. The mode transformer 190′ includes a short segment of opticalfiber with an expanded core 200 of high refractive index. The cores ofthe two fibers 3 and 8 can be expanded in order to improve the couplingefficiency between spatial modes.

The transverse transformation process is insensitive to the polarizationof the propagating pulse. However, in many applications it is necessaryto introduce different phase shifts to the different polarizations ofthe pulse. This can be desirable because the polarization of the LP₀₁mode in the single mode fiber can be different from that of the higherorder modes such as the TE₀₁ mode. FIG. 14 depicts an embodiment forsuch an application. In this embodiment a collimating lenses 88, apolarization beamsplitter 92, and a combiner 96 are conventional bulkelements. Special mirrors 100 and 102 perform the transverse modetransformation. These mirrors 100 and 102 are designed to introducephase changes to the reflected wavefronts. One way of achieving this isby etching patterns on the mirrors themselves. In another embodiment,the transverse mode transformer 28 is constructed as a single bulkcomponent 109 as shown in FIG. 15. The incident optical beam 110 issplit into two orthogonally polarized beams 111 and 113 by apolarization beamsplitter 115. Each beam is then reflected by totalinternal reflection from sides 114, and recombined at polarizationbeamsplitter 115 into a single output beam 112.

The effect of this element 109 on the polarization of the light passingthrough it is illustrated in FIG. 16. An arbitrarily polarized pulse 120is split to its two orthogonal polarization components 124 a and 124 bby the polarization beamsplitter 115 (not shown). The phase of eachcomponent 124 a and 124 b is changed by the phase elements on themirrors 114 of FIG. 15 resulting in altered components 128 a and 128 b.A polarization beamsplitter 115 (not shown) combines the components 128a and 128 b into a single annular distribution 132. The orientation ofthe phase elements on the mirrors 114 which are used to generate thealtered components 128 a and 128 b can be rotated so that all LP₁₁modescan be generated separately. As a result, only a single mode propagatesin the DCF 84 of FIG. 14. One advantage is that apolarization-maintaining fiber is not required.

If the polarization of the incident pulse is known (after a polarizer ora polarizing splitter) then it is possible to transform its polarizationto match that of the high order modes in the fiber. This polarizationtransformation can be done with a fine transverse grating. For example,the polarization of the LP₀₁ mode (the lowest order mode), which isbasically linear and uniform across the mode, can be transformed to anazimuthal one (as that of the TE₀₁) by using a transverse grating with avarying local period.

Alternatively, a birefringent element can be used. FIG. 17 represents aphysical description of the process of transforming a linearpolarization towards angular polarization by using a retardation plate.The linear polarization 140 passes through a waveplate 142 havingprimary axes oriented at an angle to the orientation of the linearpolarization. The height of the plate is designed to have an angulardependence according to the equation H₁(r,θ)=D/(2π)θ, where D is definedas the depth for which the birefringence waveplate is not changing theorientation of linear polarization. The resulting polarization 144 isshown in FIG. 17. However, this wavefront may have a residual angularphase. Therefore, another non-birefringent element 146 is used tocompensate for any residual angular phase resulting in polarization 148.This element introduces the negative angular phase. This phase can bepresented as H₂(r,θ)=F/(2π)θ, where F is calculated according to theresidual angular phase. The same effect can be achieved also by usingtwo retardation waveplates having opposite angular phases and theirprimary axis oriented at opposite angles to the linear polarization.

The transverse phase elements can be implemented in a few configurationsaccording to the requirements of the complete system. FIG. 18 representsone embodiment of a system according to the present invention which isdesigned to eliminate the sensitivity of the system to polarization modedispersion. The light propagating in a single mode fiber 3 enters acirculator 160 or a coupler (not shown). Then the light passes throughthe transverse mode transformer 162. The light is propagated as a higherorder mode in the dispersion compensation fiber 164. A Faraday mirror166 then reflects the light. After the light has passed again throughthe dispersion compensation fiber 164 and transverse mode transformer162, the circulator 160 separates the outgoing light for propagationthrough fiber 3′ from the incoming light propagating through fiber 3.

However, in many applications circulators 160 are not desired because oftheir expense and complexity. Couplers (i.e., beamsplitters) are alsoundesirable because they introduce an inherent 50% loss. FIG. 19represents a configuration in which a circulator or coupler is notneeded. The light is separated into its orthogonal polarizations by thepolarization splitter 172. Then, each polarization passes through aFaraday rotator 174 imparting a 45° polarization rotation to thepolarization and then through a phase element 178. A polarizationconserving special fiber 180 or an elliptical special fiber 180 isoriented at 45° so it is parallel to the transmitted polarization. Theinfluence of the two Faraday rotators 174 cancels the rotationintroduced by the special fiber 180. As a result, the two polarizationsreturn to their original state and are combined at the polarizer 172 inthe same orientation. The resultant outgoing light propagates throughfiber 182. As the two polarizations are counter-propagating in thespecial fiber 180, they have the same orientation. Therefore, they willbe combined without time difference.

Thus, it is intended that all matter contained in the above descriptionor shown in the accompanying drawings shall be interpreted asillustrative and not in a limiting sense. It is also to be understoodthat the following claims are intended to cover all of the generic andspecific features of the invention described herein.

I claim:
 1. A transverse mode transformer for transforming an opticalsignal propagating in an input optical waveguide, said optical signalhaving a first spatial mode having a wavefront characterized by a firstunique spatial dependence of its phase, said transverse mode transformercomprising; at least one phase element arranged to alter a phase of afirst region of said wavefront relative to an adjacent second region ofsaid wavefront; said optical signal propagating in an output opticalwaveguide in substantially a single second spatial mode having awavefront characterized by a second unique spatial dependence of itsphase.
 2. The transverse mode transformer of claim 1, wherein the firstspatial mode comprises an LP₀₁ mode.
 3. The transverse mode transformerof claim 1, wherein said single second spatial mode is an LP₀₂ mode. 4.The transverse mode transformer of claim 1, wherein said single secondspatial mode is an LP₁₁ mode.
 5. The transverse mode transformer ofclaim 1, further comprising a lens in optical communication with said atleast one phase element.
 6. The transverse mode transformer of claim 5,wherein said lens is integral with one of said at least one phaseelements.
 7. The transverse mode transformer of claim 1, wherein said atleast one phase element is arranged to alter said phase of said firstregion of said wavefront by 180° relative to said second adjacent regionof said wavefront.
 8. The transverse mode transformer of claim 1,wherein said at least one phase element is selected from the groupconsisting of a lens, a mirror, a grating, an electro-optic device, abeamsplitter, a reflective element, a graded index material, and aphotolithographic element.
 9. The transverse mode transformer of claim1, wherein said at least one phase element comprises a predeterminedbinary pattern on an end face of an optical waveguide adapted to supportsaid optical signal propagating in the first spatial mode.
 10. Thetransverse mode transformer of claim 1, wherein said at least one phaseelement comprises a predetermined binary pattern on an end face of saidoutput optical waveguide.
 11. A method for transforming an opticalsignal propagating in an input optical waveguide, said optical signalhaving a first spatial mode having a wavefront characterized by a firstunique spatial dependence of its phase, said method comprising the stepsof: providing at least one phase element arranged to alter a phase of afirst region of the wavefront relative to a second adjacent region ofsaid wavefront; directing the optical signal to propagate through saidphase element, thereby altering said phase of said first region of saidwavefront relative to said second adjacent region of said wavefront; andproviding an output optical waveguide, whereby said phase-alteredoptical signal propagates in said output optical waveguide insubstantially a second single spatial mode having a wavefrontcharacterized by a second unique spatial dependence of its phase. 12.The method of claim 11, wherein the first spatial mode comprises an LP₀₁mode.
 13. The method of claim 11, wherein said single second spatialmode is an LP₀₂ mode.
 14. The method of claim 11, wherein said singlesecond spatial mode is an LP₁₁ mode.
 15. The method of claim 11, whereinsaid at least one phase element alters said phase of said first regionof said wavefront substantially by 180° relative to said second regionof said wavefront.